according to ATI tech support, this isn't true.
When I asked them about this specifically, they told me the only thing that "clocks down" is the amount of memory.
The clocks work the same, but if you mix a 512mb card with a 1 gb card, 512 will sit idle on the 1gb card.
Nothing else is affected.
I've heard otherwise in many many places, but this comes directly from ATI techs.

you can listen to an ATi tech, or try it yourself. Overclock one of your two cards, and see if there are any changes in your benchmarks (more than +- 2%). I've tried it and believe me, there are no changes, therefore the slower card is what determines your Xfire scaling. What you said about memory is true though.